For BSD, third party distributions aren't encouraged or documented.. if you really want to do it, you'll need to learn how the particular BSD your basing your project on is built and make the changes you need yourself.

Usually a successful distribution meets a previously un-meet need or implements a new feature. Really successful distributions provide new features.
I know you said you wanted to use BSD but If you are just starting out and looking to learn you might want to try Linux from Scratch. LFS is essentially a book with cut n' paste steps that walks you through building a distribution from source code. LFS is mainly an educational tool and many of the concepts of making a build environment with compiler should carry over. I am not sure anything exists like LFS for the BSD's

For BSD, third party distributions aren't encouraged or documented.. if you really want to do it, you'll need to learn how the particular BSD your basing your project on is built and make the changes you need yourself.

Is that because the main existing 3 (Open, Net, & Free) are relatively open and can pretty much cover any requirements?

I am more interested in doing it for the learning experience & the satisfaction.

Is that because the main existing 3 (Open, Net, & Free) are relatively open and can pretty much cover any requirements?

I think a lot of it has to do with the development ideology behind the BSD's vs. Linux. Linux is, by definition, designed for building "frankenOS's", where you pick one piece here, one piece there, etc... until the whole matches your requirements. The BSD's, on the other hand, are more consistent, so rebuilding OpenBSD gives you...OpenBSD. Yeah, there are spinoffs, but most of those are package configurations on top of a (mostly) stock core.

And I'll second the mention for LFS. What that will give you is a vanilla Linux system that you can customize to whatever you want.